Second order video clipper for optical character reader

ABSTRACT

An optical character reader providing output signals proportional to the lightness or blackness of the document being scanned generates a clipping level voltage which is not a fixed percentage of the peak black minus peak white signals. The clipping level percentage varies with the density, or blackness, of the character. The black and white peak signals are applied to a differentially connected operational amplifier having an output proportional to a first multiple times the black peak minus a second multiple times the white peak, where the first and second multiples are unequal. The output of the operational amplifier is added to a signal proportional to white background thereby providing a clipping level which is a variable percentage of the difference between a black and white peak signal.

United States Patent 2,855,513 10/1958 Hamburgenetalo 3,159,8-1512/1964Groce inventor Appl. No. Filed Patented Assignee SECOND ORDER VIDEOCLIPPER FOR OPTICAL 5 Claims, 2 Drawing Figs.

[1.8. CI l78/7.l,

MCL... 1104!! 1/00 lleldolsardl 'l78/7.l, 7.2; 250/214, 2l9'CR; 346/146.3

References Cited UNITED STATES PATENTS 12 rune SUI/MIG [WISH aucx 11PEAK R 06mm s OTHER REFERENCES ANALOG COMPUTATION Jackson McGraw-HillBook Company, 1960 pp. 47.

Primary Examiner-Robert L. Griffin Assirtant Examiner-Donald E. StoutAttorney-Sughrue, Rothwell, Mion, Zinn & Macpeak ABSTRACT: An opticalcharacter reader providing output signals proportional to the lightnessor blackness of the'docu ment beingscanned generates a clipping levelvoltage which is not a fixed percentage of the peak black minus peakwhite signals. The clipping level percentage varieswith the density, orblackness, of the character. The black and white peak signals areapplied to a differentially connected operational amplifier having anoutput proportional to a first multiple times the black peak minus asecond multiple times the white peak, where the first and. secondmultiples are unequal. The output of the operational amplifier is addedto a signal proportional to white background thereby providing aclipping level which is a variable percentage of the difference betweena black and white peak signal.

R7 8 {Z0 28 fl/HTER 511mm 7 30 E FOLLOWER F OLLU/IER 38 R6 32 c1 ,22

SECOND ORDER VIDEO CLIPPER FOR OPTICAL CHARACTER READER BACKGROUND OFTHE INVENTION In optical character recognition systems, a document isscanned and electrical signals are developed having anamplitudeproportional to the blackness of the document intercepted bythe scanning means. As the character density or blackness increasesthere is less light reflected into the photomultiplier circuit of thescanning mechanism making the output electrical signal more positive.The output electrical signals are applied to a threshold means whicheffectively decides whether the scanning mechanism is presently scanninga black character or a white background region. Information signalsidentifying the position of the scanning beam and the black charactersignals out of the threshold means are applied to recognition logicwhich operates to identify the character being scanned. In such systems,it is necessary to provide a reference level voltage, referred to as theclipping level voltage or clipping level, for comparison with theelectronic signals to determine whether or not the electronic signal isto be presently identified as a black signal on a white signal. (Theterms black" and "white" as used herein refer to the character andbackground, respectively, and although the character is normally blackand the background normally white, it is not intended that the wordsblack and white" as used herein be strictly limited to the colors blackand white.)

In the prior art, the clipping level is a fixed percentage of the blackpeak minus white peak. That is, a system was provided for detecting themost positive amplitude, corresponding to the black peak, and the leastpositive amplitude, corresponding to the white peak, over apredetennined period of time and generating a clipping level in responseto these two signals. In such systems, the clipping level, althoughvarying with a change in the density or blackness of the characters,remained a fixed percentage of the black peak minus white peak. In somesystems, in order to cope with specific problems, such as very dark orvery light characters, various shifts or different percentages aresometimes switched into the clipping level generator. The latter schemehas been less than satisfactory for a broad range of densities ofcharacters.

Generally, when the characters are very dense, such as those resultingfrom a typewriter having a new ribbon, the document will contain a lotofsmudges resulting in a high level of background noise (unwantedsignals). On the other hand, for a very old typewriter ribbon thedocument will contain low density characters and very little backgroundnoise. If a clipping level generator is set at a fixed percentage ofblack minus white sufficient to eliminate the high level noise in theformer case, then it may be insufficient to detect the low densitycharacters in the latter case. The reverse is also true.

SUMMARY OF THE INVENTION In accordance with the present invention, aclipping level is generated having a clipping level percentage which isa function of the character density. That is, for very dense characters,resulting in high amplitude black level signals, the clipping level willbe a relatively high percentage of the black minus white peaks, whereasfor a low density character the clipping level will be a relatively lowpercentage of the black minus white peaks. This is accomplished byconnecting the black and white peaks to a differentially connectedoperational amplifier having external resistors selected to provide adifference voltage proportional to the difference between a firstmultiple times the black peak and a second multiple times the whitepeak. The latter signal is added to a white background signal to providethe clipping level.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partial block diagram andpartial schematic diagram illustrating a preferred embodiment of thepresent invention operating in conjunction with an optical scanningmechanism.

FIG. 2 illustrates examples of waveforms produced by an optical scannermechanism.

DETAILED DESCRIPTION OF THE DRAWINGS The invention will be described inconjunction with an optical character reader which operates in thefollowing mode: The first character detected on a line is curvedfollowed by the optical scanner for the purpose of normalization. Onlythe first character is curved followed and all of the remainingcharacters in the line are scanned by the recognition scan. Manydifferent types of recognition scans are well known in the art and theparticular type used is not important to the present invention. Duringthe curve follower normalization scan, the black and white peaks aredetected to provide the necessary difference signal for generating theclipping level voltage. The difference signal which is generated is heldduring the time that the remainder of the characters in a line arescanned.

It should be noted that although the invention is described inconjunction with an optical character reader operating in accordancewith the above format, the format in itself forms no part of the presentinvention.

Referring to FIG. I, there is shown a scanning mechanism illustratedgenerally at 10, a white peak detector 12, a black peak detector 14, apair of operational amplifiers l6 and 32, a pair of emitter followers 20and 28, a capacitor 22, and a track and hold circuit 30. The output ofscanning mechanism 10, which is connected to the peak detectors l2 and14 is an electronic signal having an upper level proportional to theblackness of the characters in the document being scanned and a lowerlevel proportional to the whiteness of the document background. In orderto detect black and white peaks during the curve follower scan, it iswell known in the art to use amplitude demodulators for the peakdetectors. Thus, white peak detector 12 may be a conventional amplitudedemodulator which detects the lower envelope of the input frequency andblack peak detector 14 may be a conventional amplitude demodulator whichdetects the upper envelope of the input frequency.

During the curve follower scan, as is well known in the art, the outputof scanning mechanism 10 will have the form of any high frequencysignal, e.g., 50 kc., having an upper amplitude envelope proportional tothe blackness of the character which is being scanned and a loweramplitude envelope proportional to the whiteness of the documentadjacent the character being scanned. If the character is fairly uniformand there are substantially no smudges around the character, the upperand lower envelopes will not vary. A typical signal out of the scanningmechanism during a curve follower scan is illustrated in waveform (a) ofFIG. 2, wherein the frequency of the signal 40 is dependent upon thecurve follower frequency, the upper amplitude envelope, shown by dottedline 42, is proportional to the blackness of the character beingscanned, and the lower amplitude envelope, illustrated by dotted line 44is proportional to the white background of the document adjacent thecharacter being scanned. Since peak detector 12 detects the lowerenvelope, a signal level which is the same as the signal level forbroken line 44 will appear at the output of peak detector 12. Also,since peak detector 14 detects the upper envelope of the input signal,the output level from peak detector 14 will be the same as broken line42 of waveform (a). The output levels will hereinafter be referred asthe white peak and black peak, respectively. The manner just describedfor detecting the white and black peaks in an optical scanning mechanismis not critical to the present invention, and, in fact, any priorapparatus and method for detecting the white and black peaks willoperate with the present invention, which will now be described.

The outputs from the peak detectors l2 and 14 are connected to adifferential amplifier means comprising operational amplifier l6, andresistors R thru R Operational amplifiers are well known in the art andthe gain of the overall amplifier is determined by the externalresistances connected thereto. For an operational, differentiallyconnected amplifier, such as operational amplifier 16, having externalresistors R, through R connected as shown, the gain expression is:

R3(R1+R4) 7 R (R2+R3) Black R1 h1te,

where Black is the signal from the peak detector 141, White is thesignal from peak detector 12, both detected during the normalizationscan. E is the output voltage from amplifier 16. R, and R re the inputresistors. R is the reference resistor.

dhisfi sfsedbas rs ist lt is known in the prior art to make R,=R,-R andR R R' thus reducing the above expression to:

E,,='/R (Black White).

In order to generate the clipping level voltage, E the output E is addedto the white background level resulting in the expression:

R E11,: F (Black White) \Vhite'.

second multiples are unequal. As can be seen, therefore, this is anonlinear function of the difference between the black level and thewhite level. When an output of this nature is added to the whitebackground voltage, the result is a clipping level voltage having apercentage of black minus white which varies nonlinearly with thedensity of the characters. For a specific example let the resistors Rthrough R, have the following values:

R %=4K; R =12K and R =2ahK The gain expression for the amplifier thusbecomes:

In the latter expression, the first multiple is 2.25 and the secondmultiple is l. The expression for E becomes:

The following tables illustrate how the percentage of (B-W), which is Evaries for different values of the black peak level. In the tables thewhite peak level is assumed constant. In Table l, the white peak levelis assumed to be at 4.0 volts and in Table II, the white peak level isassumed to be at 3.0 volts.

TABLE I %(2.25B W) Percent 2.2513 2.25B W %(2.25B- W) +W EcL Black pcakiBLE 11 =3.0 volts] 2 7 5B-W Percent 2 25B- W 2 ECL 2.2512- W 2253- W 2+W Black peak:

Hereinafter, the letters W and B will be used in the equations toexpress the terms White and Black, respectively. The above expressionfor E results in a clipping level voltage which varies with the densityof the characters but remains a function of a fixed percentage of thedifference between the black and white levels. That percentage isdetermined by the ratio of R to R, and can be altered by changing thisratio. However, as pointed out above, it has been discovered that it ispreferable to have a clipping level voltage which is not a fixedpercentage of the difference between the black and the white levels.

in accordance with the present invention, the feedback resistance R ismade unequal to the reference resistance R TABLE III %(2.1B W) Percent2.1XB 2.1XB- W %(2.1BW) +W Em,

Black peak:

resulting in an output E which is proportional to the difference betweena first multiple times the black level and a There may be many methodsand circuits for adding the output E hereinafter referred to as thedifference output, to

second multiple times the white level where the fi t d the whitebackground signal, and one of these methods and circuits is illustratedin FIG. 1. it should be understood that the method of deriving the whitebackground level and the particular circuitry used to add the differencesignal to the white background signal is not critical to the presentinvention.

As discussed above, the invention is assumed to be operating, in H6. 1,with an optical character recognition system that performs a curvefollowing normalization scan on the first character of the line only,and performs a different type of recognition scan on all of thecharacters in the line. During the curve follower mode, switch 18 isclosed, thus allowing the difference voltage from the differenceamplifier means, comprising operational amplifier l6 and the resistors Rthrough R to be applied through the emitter follower 20 to storagecapacitor 22 which has one terminal connected to ground at 20. Thedifference voltage stored on capacitor 22 is applied through emitterfollower 28 to track and hold circuit 30. The track and hold circuitperfonns the function of a long period storage capacitor. That is, theinput from capacitor 22 is held at the output of track and hold circuit30 for a long period of time, sufficient to allow the scanner tocomplete the recognition scan of an entire line of characters.

Track and hold circuits are known in the prior art and examples of suchcircuits are described in commonly assigned US. Pat. application Ser.No. 619,226, filed Feb. 28, 1967, for High Speed Registration Techniquefor Position Code Scanning, by William W. Hardin, et al.

The difference voltage, appearing in the output of track and holdcircuit 30, is applied to a summation amplifier means comprisingdifferentially connected operational amplifier 32, input resistors R andR reference resistor R and feedback resistor R Assuming that resistors Rthrough R are all equal and for a specific example, assuming they areequal to 2K, the output voltage E at terminal 34 will be equal to thesum of the voltages at terminals 38 and 36.

The voltage at terminal 36 is the white background voltage and in thespecific example described herein is derived from the white peakdetector 12. An example of the type of signal which appears at theoutput of scanning mechanism 10 during the recognition scan isillustrated by waveform (b) of FIG. 2. ln waveform (b), the lower levelrepresents the white background level and the pulses representexcursions of the scanning beam across a portion of a black character.For any given document, the white level of waveform (a) will be the sameas the white level of waveform (b). instantaneous changes in the lowerlevel of the waveforms may result from smudges on the document. Sincethe white peak detector 12 is an amplitude demodulator, and further,since the excursion of the scanning beam across the black characterresults in a short, high frequency pulse, the output of the peakdetector will be equal to the lower level ofwaveform (b).

While the invention has been particularly shown and described withreference to a preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention.

What I claim is:

1. In an optical scanning system of the type which scans a documentcontaining characters and develops electrical signals proportional tothe degree of blackness and whiteness intercepted by the scan, aclipping-level generator comprising:

amplifier means responsive to first and second input signalsrepresenting peak blackness and peak whiteness of said electricalsignals;

means coupled to said amplifier means for generating an output signalwhich is a nonlinear function of the difference between said firstsignal and said second signal, and

means for adding said output signal to a signal representing the whitebackground of said document.

2. A clipping-level generator as claimed in claim 1 wherein saidamplifier means is an operational amplifier having first and seconddifferential input terminals and an output terminal, and wherein saidmeans for generating said output signal is coupled to at least one ofsaid differential input terminals.

3. A clipping-level generator as claimed in claim 2 wherein said outputsignal is proportional in amplitude to the difference between a firstmultiple of said first signal and a second multiple of said secondsignal, said first and second multiples being unequal.

4. A clipping-level generator as claimed in claim 3 wherein said meansfor generating said output signal comprises:

a feedback resistor connected between one of said differential inputsand said output terminal,

a reference resistor connected between the other of said differentialinputs and a reference potential, said reference resistor being unequalto said feedback resistor, and

a pair of input resistors connected to said differential input terminalsfor connecting said first and second signals to said differential inputterminals.

5. A clipping-level generator as claimed in claim 4 wherein saidreference resistor is greater than said feedback resistor.

1. In an optical scanning system of the type which scans a documentcontaining characters and develops electrical signals proportional tothe degree of blackness and whiteness intercepted by the scan, aclipping-level generator comprising: amplifier means responsive to firstand second input signals representing peak blackness and peak whitenessof said electrical signals; means coupled to said amplifier means forgenerating an output signal which is a nonlinear function of thedifference between said first signal and said second signal, and meansfor adding said output signal to a signal representing the whitebackground of said document.
 2. A clipping-level generator as claimed inclaim 1 wherein said amplifier means is an operational amplifier havingfirst and second differential input terminals and an output terminal,and wherein said means for generating said output signal is coupled toat least one of said differential input terminals.
 3. A clipping-levelgenerator as claimed in claim 2 wherein said output signal isproportional in amplitude to the difference between a first multiple ofsaid first signal and a second multiple of said second signal, saidfirst and second multiples being unequal.
 4. A clipping-level generatoras claimed in claim 3 wherein said means for generating said outputsignal comprises: a feedback resistor connected between one of saiddifferential inputs and said output terminal, a reference resistorconnected between the other of said differential inputs and a referencepotential, said reference resistor being unequal to said feedbackresistor, and a pair of input resistors connected to said differentialinput terminals for connecting said first and second signals to saiddifferential input terminals.
 5. A clipping-level generator as claimedin claim 4 wherein said reference resistor is greater than said feedbackresistor.